1. Field of the Invention
The present invention relates to an electronic timer having an aging device that is on or off for a specific time because of the charge accumulated in it. The invention relates to a system LSI that has such an electronic timer.
2. Description of the Related Art
Any system LSI available at present incorporates a timer module that safeguards the system LSI against an abrupt power failure due to, for example, a blackout. The timer module is composed of a micro-battery, a quartz-crystal resonator, and a timer controller. The timer controller is provided in the system chip. The micro-battery and the quartz-crystal resonator, which are arranged outside the system chip, are particularly expensive, raising the manufacturing cost of the system LSI.
One of the methods of solving this problem is to fabricate, in the system LSI chip, an electronic device that is an integrated circuit capable of informing, in a battery-less mode, how long the system LSI has been exposed to a blackout. The inventors hereof have proposed a solid state aging device (SSAD) that can be integrated in the system LSI and can control, without batteries, the time for which the system LSI can operate. (See JP-A 2004-172404(KOKAI).) This aging device is designed to read digital data representing transition of the on/off state. Therefore, the aging device must have a great number of aging-device cells if it is to operate as a timer.
A method of reading time from a very small change in a current at on level is disclosed in, for example, U.S. patent Ser. No. 09/703,344. This method requires large capacitors to stabilize changes in time. The aging is inevitably determined by the thin part of the tunnel insulating film of each large capacitor. Hence, it is difficult to control the difference between individual devices. A method of disclosed, in which SONOS is used to read time from a very small current at on level. However, this method can hardly control the difference between individual devices, either, because the traps in the insulating film are used in the method.
As indicated above, the conventional system LSI needs to have a timer module for measuring the time at which the LSI should start operating normally after the interruption of power supply. An aging device that operates without batteries may be used as an electronic timer. In this case, the device needs to have a great number of aging-device cells. This results in an increase in the manufacturing cost of the system LSI.